The verse "In my heart I have hidden your word" means that if a person internalizes the Torah and has it in their heart, the evil inclination does not control them and cannot harm them. As it says in Proverbs 8:12, "I, wisdom, dwell with prudence." Where wisdom resides, the evil inclination does not reign. Similarly, in Psalm 37:31 it says, "The Torah of his God is in his heart; his steps will not falter." And as Moses says in Deuteronomy 11:18, "Therefore, impress these My words upon your heart and upon your soul." Therefore, it is said, "In my heart I have hidden your word."